U.S. patent number 4,842,367 [Application Number 07/041,099] was granted by the patent office on 1989-06-27 for optoelectronic directional coupler for a bias-free control signal.
This patent grant is currently assigned to Telefonaktiebolaget L M Ericsson. Invention is credited to Anders G. Djupsjobacka.
United States Patent |
4,842,367 |
Djupsjobacka |
June 27, 1989 |
Optoelectronic directional coupler for a bias-free control
signal
Abstract
An optoelectronic directional coupler (1) has in a coupling area
two parallel, coupled lightwave conductors (6a and 6b) with a
length (L) as well as electrodes (5a and 5b). Each of the coupled
lightwave conductors has one end connected to its individual output
(7a and 7b) of the directional coupler (1). At their other ends the
coupled lightwave conductors are each connected to its extra
individual lightwave conductor (8a and 8b), the conductors (8a and
8b) being connected to the directional coupler input (9) via a fork
branch. An incoming lightwave (P) is divided into two partial
lightwaves (P1 and P2) by the extra wave conductors (8a and 8b).
The partial lightwaves are in phase with each other and have the
same effect in relation to each other at the inputs to the coupled
lightwave conductors (6a and 6b). The partial lightwaves can be
switched to either of the outputs (7aand 7b) with the aid of a
control signal (S) connected between the electrodes (5a and 5b).
The directional coupler has the advantage that the control signal
(S) is a pure alternating voltage.
Inventors: |
Djupsjobacka; Anders G. (Solna,
SE) |
Assignee: |
Telefonaktiebolaget L M
Ericsson (Stockholm, SE)
|
Family
ID: |
20364550 |
Appl.
No.: |
07/041,099 |
Filed: |
April 22, 1987 |
Foreign Application Priority Data
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May 16, 1986 [SE] |
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86022340 |
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Current U.S.
Class: |
385/41;
385/22 |
Current CPC
Class: |
G02F
1/3134 (20130101) |
Current International
Class: |
G02F
1/29 (20060101); G02F 1/313 (20060101); G02B
006/26 () |
Field of
Search: |
;350/96.11,96.12,96.13,96.14,96.15 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2533714 |
|
Mar 1984 |
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FR |
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52-32347 |
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Mar 1977 |
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JP |
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56-147122 |
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Nov 1981 |
|
JP |
|
WO86/01907 |
|
Mar 1986 |
|
WO |
|
Other References
Schmidt et al., "Electro-Optically Switched Coupler . . .
LiNBO.sub.3 Waveguides", Appl. Phys. Lett., vol. 28, No. 9, May
1976, pp. 503-506. .
Kogelnik et al., "Switched Directional Couplers with Alternating
.DELTA..beta.", IEEE J. of Quantum Electronics, vol. QE-12, No. 7,
Jul. 1976, pp. 396-401. .
Cross et al., "Optically Controlled . . . Switch", IEEE J. of
Quantum Electronics, vol. QE-14, No. 8, Aug. 1978, pp. 577-580.
.
Gee et al., "Traveling-Wave Electrooptic Modulator", Applied
Optics, vol. 22, No. 13, Jul. 1983, pp. 2034-2037. .
Introduction to Integrated Optics, Plenum Press (New York, London),
Chap. 14, "Acousto-Optical Interactions in Guided Wave Structures",
E. G. H. Lean, pp. 411-413, 421-431, 437-441, 445-449, 454-455,
(1974). .
Cross et al., "Microwave Integrated Optical Modulator", Applied
Physics Letters, 44 (5), Mar. 1, 1984, pp. 486-488. .
S. Thaniyavarn, "A Novel .DELTA..beta. Phase Reversal Mach-Zehnder
Interferometer", TRW Electro Optics Research Center, 02/26-2/28/86.
.
R. Alferness, "Guided-Wave Devices for Optical Communication", IEEE
Journal of Quantum Electronics, vol. QE-17, No. 6, Jun. 1981, pp.
946-959..
|
Primary Examiner: Lee; John D.
Attorney, Agent or Firm: Burns, Doane, Swecker &
Mathis
Claims
I claim:
1. Optoelectronic directional coupler including a) two mutually
spaced, coupled led lightwave conductors extending in a coupling
area, the conductors each being connected to a waveguide output on
the directional coupler and two electrodes in the coupling area,
with the aid of which the optical coupling between the coupled
conductors can be acted on with the aid of a control signal, b)
extra lightwave conductors, of which at least one is in
communication with a wave conductor input on the directional
coupler, the extra lightwave conductors being arranged such that an
incoming lightwave on the directional coupler wave conductor input
is divided between the extra lightwave conductors into two partial
lightwaves, each having substantially the same power and being in
phase with the other, or being phase-shifted half a revolution in
relation to the other, characterized in that the coupled lightwave
conductors (6a, 6b; 22a, 22b; 33a, 33b; 43a43b) are each connected
to one of the extra lightwave conductors (8a, 8b; 36a, 36b; 46a,
46b) and in that the incoming lightwave (P) can be switched between
the outputs (7a, 7b; 35a; 35b, 45a, 45b) of the directional coupler
(1;30;40) by the partial lightwaves (P1,P2;P4,P5;P6,P7) being
switched between the coupled lightwave conductors with the aid of
the control signal (S) connected to one of the electrodes (5a; 34a;
44a), the signal assuming a positive (+VO) or a negative (-VO)
potential on switching, in relation to a reference potential to
which the other electrode (5b; 34b; 44b) is connected, the positive
potential (+VO) having substantially the same numerical value as
the negative potential (-VO).
2. Optoelectronic directional coupler as claimed in claim 1, where
each of the coupled lightwave conductors has a continuous
electrode, characterized in that the electrodes have a length L for
which the relationship L=12.sqroot.2 Lc is applicable where Lc is
one coupling length for the coupled lightwave conductors (6a, 6b;
33a, 33b; 43a43b).
3. Optoelectronic directional coupler as claimed in claim 1, where
each of the coupled lightwave conductors has an electrode which is
divided into two substantially equally as long sections,
characterized in that the electrodes have a length L1 for which the
relationship L1=1.85.times.Lc is applicable, where Lc is one
coupling length for the coupled lightwave conductors (22a, 22b).
Description
TECHNICAL FIELD
The invention relates to an optoelectronic directional coupler for
a bias-free control signal, the coupler including (a) two coupled
lightwave conductors which are situated at mutual spacing and
extend in a coupling area and are each connected to a wave
conductor output on the directional coupler, (b) electrodes in the
coupling area with the aid of which the optical coupling between
the coupled lightwave conductors can be acted on with the aid of
the control signal.
BACKGROUND ART
Optoelectronic directional couplers are used in many applications
in modulating a lightwave or switching light signals in such as
optical communication systems. A description of directional
couplers is to be found in IEEE Journal of Quantum Electronics,
vol. QE-12, No 7, July 1976, H. Kogelnik and R.V. Schmidt:
.cent.Switched Directional Couplers with Alternating
.DELTA..beta.". The directional couplers are intended for two
coupled lightwave conductors, which are generally connected to
their individual inputs and outputs of the coupler. The optical
coupling between the lightwave conductors can be acted on with the
aid of an electrical signal which is connected to electrodes at the
coupled lightwave conductors. The directional couplers of the prior
art have a transfer function according to which it is required that
the electrical signal has a direct voltage level, an electrical
bias, about which the modulating signal varies. In high frequency
modulation, with modulation frequencies of about 5 GHz or higher,
there are problems in keeping this level constant. A varying bias
can cause an optical signal to be incompletely coupled and sent
from both outputs of the directional coupler. A Mach-Zender type of
bias-free modulator is illustrated in Appl. Phys. Lett 43 (11), Dec
1983, C.M. Gee, G.D. Thurmond and H.W. Yen: "17 GHz band-width
electro-optic modulator". This modulator has wo wave conductors
between which an incoming lightwave is divided, the lightwave being
relatively phase-shifted in the respective wave conductors. The
modulator has the disadvantage that it only has one input and one
output, so that there lacks the availability of switching a signal
between two outputs.
DISCLOSURE OF INVENTION
The above-mentioned problems are avoided, in accordance with the
invention, by a directional coupler which has two outputs, and also
has no direct voltage component in its control signal.
BRIEF DESCRIPTION OF DRAWINGS
An embodiment of the invention will now be described in more detail
below in connection with a drawing, where
FIG. 1 illustrates an inventive direction coupler in perspective
from above,
FIG. 2 is a cross section through the coupler of FIG. 1,
FIG. 3 is a plan view from above of the coupler in FIG. 1 showing
it connected to a light source and a modulating voltage source,
FIG. 4 is a diagram of a transfer function for the coupler in FIG.
1,
FIG. 5 shows diagrams of a modulating electrical signal and a
corresponding modulated light signal,
FIG. 6 is a plan view from above of the coupler according to FIG. 1
with an alternative implementation of the electrodes,
FIG. 7 is an alternative implementation of a directional coupler in
accordance with the invention, and
FIG. 8 is a still further alternative implementation of an
inventive directional coupler.
BEST MODES FOR CARRYING OUT THE INVENTION
An embodiment of a directional coupler 1 in accordance with the
invention is illustrated in FIG. 1. The coupler includes a wafer 2
of optoelectronic material, e.g. lithium niobate, which has wave
conductor means 4 and electrodes 5a and 5b at its upper planar
surface 3. The wave conductor means can be achieved by a process
such as diffusing titanium into the wafer 2 to a desired depth. In
a coupling area the wave a conductor means 4 has two parallel,
coupled wave conductors 6a and 6b with a length L and situated at a
mutual spacing d. This spacing is selected such that the light in
one coupled wave conductor acts on the other coupler wave conductor
and can migrate over to it. The coupling can be acted on with the
aid of the electrodes 5a and 5b, which have the length L and extend
along their respective coupled wave conductor 6a and 6b. At their
ends the wave conductors are connected to their respective outputs
7a and 7b on the directional coupler 1. At their other ends the
wave conductors 6a and 6b have their respective input connected to
extra lightwave conductors 8a and 8b. The latter are connected to
each other and to a wave conductor input 9 on the directional
coupler 1 to form a fork branch.
A cross section of the directional coupler 1 is illustrated in FIG.
2 with the diffused, coupled lightwave conductors 6a and 6b and the
electrodes 5a and 5b on the upper planar surface of the wafer
2.
The directional coupler 1 is shown in plan in FIG. 3. The wave
conductor input 9 is connected by an optical fibre 10 to a
schematically illustrated laser 11, which sends a light-wave P to
the coupler. A conductor 12 connects the electrode 5a to a
schematically illustrated signal source 13, which sends a controls
signal S to the electrode 5a. The electrode 5b is connected to
earth potential, and both electrodes, which are coupled as
so-called traveling wave electrodes, are terminated reflection-free
via a resistor R connected between them.
The incoming lightwave P is divided in the fork branch between the
extra lightwave conductors 8a and 8b into two partial lightwaves P1
and P2. At the inputs to the coupled wave conductors 6a and 6b the
partial lightwaves are in phase with each other, and the branch is
formed such that the partial wave P1 has substantially the same
power as the partial wave P2. If the control signal S=O, the
partial lightwaves travel along the coupled wave conductors 6a and
6b and are sent from the outputs 7a and 7b. If the controls signal
assumes a value S=VO, the coupling is acted on between the coupled
lightwave conductors so that the partial lightwave P2 is switched
from coupled lightwave conductors 6b to the coupled lightwave
conductor 6a. An outgoing ligh signal P3 is sent from the wave
conductor output 7a with the whole of the power of the incoming
lightwave P. For the opposite sign of the control signal, S=-VO,
the whole power of the incoming lightwave P is sent from the wave
conductor output 7 b.
The transfer function A of the directional coupler 1 is given in
FIG. 4, which illustrates in more detail how the power of the
incoming lightwave P is distributed between the wave conductor
outputs 7a and 7b in response to the strength of the control signal
S.
It has been described above how the incoming lightwave P is divided
and switched between the coupled lightwave conductors. For the
signal voltage S=.+-.VO the directional coupler 1 sends the
incoming lightwave P completely from the output 7a or 7b. This
takes place, however, solely with the provision that the length L
of the coupling area is in a given relationship to a coupling
length Lc for the coupled lightwave conductors 6a and 6b. By the
coupling length Lc is intended here the length along the coupled
lightwave conductors which is required for a lightwave in one
conductor to be entirely switched over to the other conductor when
the signal S=O. The condition applying for the directional coupler
in FIG. 3 is that L=1.sqroot..times.2.times.Lc.
An example is illustrated in FIG. 5 of how the lightwave P from the
laser 11 is modulated with the aid of the control signal S. Here
the FIG. 5a illustrates in a diagram how the control signal S
varies with the time T between the voltages +Vo and -VO. The
control signal represents information in the form of logical ones
and zeros, as marked under the diagram. FIG. 5b illustrates the
corresponding light signal P3, which is sent from the wave
conductor output 7a.
An inventive directional coupler 20 is illustrated in FIG. 6, and
is provided with lightwave conductors 21 of the same implementation
as for the directional coupler 1 described above. Two parallel,
coupled lightwave conductors 22a and 22b of a length L1 extend over
a coupling area. These coupled conductors have one end connected to
the outputs 23a and 23b of the directional coupler 20, and at their
other ends they are connected to each other and to the input 24 of
the coupler via a fork branch. In the coupling area, the
directional coupler 20 has electrodes 25a and 25b, which are
divided into sections, there being two sections in the embodiment
here. A more detailed description of this type of electrode is to
be found in the article by H. Kogelnik and R.V. Schmidt cited
above. By the division of the electrodes into sections, an
electrical adjustment of the coupling between the coupled
conductors 22a and 22b can be achieved. Accordingly an almost
complete switching over of light from one or the other output is
obtained, even if the length L1 deviates from the desired length,
e.g. due to deficient manufacturing accuracy. For the directional
coupler 20 the relationship: L1=1.85.times.Lc, where Lc is the
above-mentioned coupling length.
An alternative embodiment of a directional coupler 30 in accordance
with the invention is illustrated in FIG. 7. The coupler includes a
wafer 31 of optoelectronic material with a wave conductor means 32.
In a coupling area the wave conductor has two parallel, coupled
lightwave conductors 33aand 33b at a mutual spacing of d2 and with
a length L2. The switching coupling between the coupled lightwave
conductors can be acted on with the aid of two electrodes 34a and
34b, each extending along their respective conductors 33a and 33b.
The latter are at one end connected to their individual outputs 35a
and 35b on the directional coupler. At their other ends the
conductors each have their input respectively connected to an extra
lightwave conductor 36a and 36b. The latter are parallel and extend
a length L3 in an extra coupling area in which light can be
switched between the extra lightwave conductors. This switching can
be acted on with the aid of extra electrodes 37a and 37b, which
extend in the extra coupling area along their respective lightwave
conductors 36a and 36b. These conductors are connected by their
respective wave conductor inputs 38a and 38b on the directional
coupler 30. The laser 11 is connected to the wave conductor input
38b via the optical fibre 10, and sends the lightwave P to this
input. The lightwave P travels along the extra lightwave conductor
36b and is switched to the extra lightwave conductor 36a. The
switching is controlled with the aid of a direct voltage source 39
connected between the electrodes. It is here desirable that the
switching gives a partial lightwave P4 and P5, respectively, at the
inputs to the coupled lightwave conductors 33a and 33b, so that the
partial lightwave P4 substantially has the same power as the
partial lightwave P5 and the mutual phaseshift between the partial
waves is substantially 180.degree.. A calculation of the coupled
oscillations occuring between the extra lightwave conductors 36a
and 36b illustrates that it is possible to select the extra
coupling area length as L3, the distance d2 between the extra
lightwave conductors and the output voltage U of the direct voltage
source in such a way that this desire is met. The electrode 34a is
connected to the signal source 13, the electrode 34b is connected
to earth potential, and both electrodes are coupled as travelling
wave electrodes and connected to each other via the resistor R. The
signal source 13 sends the controls signal S for controlling the
partial lightwaves P4 and P5 between the outputs 35a and 35b of the
directional coupler 30 in a manner described above in connection
with FIG. 3.
A still further alternative embodiment of a directional coupler 40
in accordance with the invention is illustrated in FIG. 8. Similar
to the embodiments described above, the directional coupler 40 has
a wafer 41 of optoelectronic material having on its upper surface a
wave conductor means 42. In a coupling area, the wave conductor
means has two parallel, coupled lightwave conductors 43a and 43b
with a length of L4 with a mutual spacing of d4, and also
electrodes 44a and 44b. The conductors 43a and 43b are at one end
each connected to their outputs 45a and 45b on the directional
coupler and at their other ends they have an input connected to
extra lightwave conductors 46a and 46b. These extra conductors are
parallel and extend in an extra coupling area of a length L5. Half
way between the extra conductors 46a and 46b there extends a light
distributing lightwave conductor 47 which is connected to the input
48 on the directional coupler 40. The lightwave P coming to this
input is distributed by coupled oscillations of the light
distributing lightwave conductor 47 between the extra lightwave
condutors 46a and 46b. The whole of the light energy in the light
wave P is switched to the extra lightwave conductors, which send
respective partial lightwaves P6 and P7 at the inputs to the
coupled lightwave conductors 43a and 43b. These partial lightwaves
are in mutual phase, and in relation to each other they have
substantially the same power. The partial lightwaves can be coupled
between the outputs 45a and 45b of the directional coupler 40 with
the aid of the electrodes 44a and 44b, as described in connection
with FIG. 3 above.
The inventive directional couplers described above have the
advantage that they have a relatively high upper boundary
frequency, in the order of magnitude 7 GHz, at a relatively low
modulation voltage S. This modulation voltage does not have a
direct voltage component and is therefore comparatively simple to
generate. The embodiments according to FIGS. 3 and 8 have the
advantage that the coupling area L and L4, respectively, is short,
so that the optelectronic wafer 2 and 41 is small and there are no
extra electrodes which are biased with a direct voltage. Their
disadvantage is that they only have one input 9 and 48,
respectively, so that it is not possible to cross-couple two light
signals with the air of one directional coupler. The embodiment
according to FIG. 7 has the advantage that it can have two inputs
38a and 38b. Its disadvantage is that it requires a direct voltage
and that it has two coupling area L2 and L3 with electrodes, which
means that the directional coupler 30 requires a large
optelectronic wafer 31.
In the illustrated embodiments the directional coupler has a wafer
2, 31, 41 with an orientation of the crystalline axes such that the
light is propagated in the direction of the optical axis. It is
possible to apply the invention on directional couplers with an
orientation of the crystalline axes which deviates from this.
However, the electrodes then have an implementation adjusted to
this deviating crystalline orientation and an appearance deviating
from the electrodes illustrated in the Figures.
* * * * *